1. Field of the Invention
The present invention relates to a process for the preparation of vinyl polymers by bulk or solution polymerization and an apparatus for practicing this process. More particularly, the present invention relates to a process for preparing vinyl polymers using a reaction vessel of the complete mixing type, in which polymerization is carried out while a portion of the polymer-containing liquid is withdrawn from the reaction vessel and recycled to the polymerization starting material. The invention also relates to apparatus for practicing this process.
2. Description of the Prior Art
The technique of preparing vinyl polymers, for example, methyl methacrylate polymers, by bulk or solution polymerization is superior to the conventional suspension polymerization technique in that there is no need for waste water treatment and the polymerization can be conducted continuously with ease. However, this process has the drawback that if the polymerization ratio, i.e., the ratio of the formed polymer to the starting monomer, is increased, the viscosity of the polymerization mixture increases drastically and consequently, in order to remove heat of polymerization and handle the reaction mixture satisfactorily, special measures have to be taken, resulting in increased equipment cost. Especially in bulk polymerization of methyl methacrylate an acceleration of the polymerization known as the "gel effect" occurs; and if the viscosity of the polymerization mixture is high, it becomes difficult to control the polymerization. In order to eliminate this disadvantage, a method has recently been proposed in which the viscosity of the polymerization mixture is reduced and the monomer is polymerized at a relatively low polymerization ratio (see Japanese Patent Publication No. 32665/77, U.S. Pat. No. 3,900,453 and U.S. Pat. No. 3,968,509). This polymerization process is of the complete mixing type in which the reaction mixture in the reaction zone is agitated and substantially homogeneously mixed, and the reaction is carried out while maintaining reaction conditions such as the reaction temperature, the initiator concentration, the polymerization regulator concentration, the polymerization ratio and the viscosity of the reaction mixture at substantially constant levels. Accordingly, in continuous polymerization of this complete mixing type, fresh monomer containing a polymerization inhibitor and a polymerization regulator, which is continuously introduced, should be mixed promptly and homogeneously with the reaction mixture in the reaction vessel.
However, in order to increase the manufacturing rate, the viscosity of the reaction mixture in the reaction vessel of the complete mixing type is increased to a level of scores of poises to several thousand poises. On the other hand, the viscosity of the monomer containing polymerization initiator and polymerization regulator is as low as about 0.01 poise. Accordingly, various problems arise because of the great difference in viscosity. For example, the efficiency of mixing and agitation is lowered and good reaction control cannot be attained. Furthermore, the polymer concentration is non-uniform because of insufficient mixing, resulting in formation of a heterogeneous polymer of lower quality.
Hence, a need has continued to exist for a method of carrying out continuous polymerization of vinyl monomers which provides efficient polymerization combined with good control of the reaction conditions for production of high quality polymer.